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A class II KNOX gene, KNOX4, controls seed physical dormancy.

ABSTRACT: Physical dormancy of seed is an adaptive trait that widely exists in higher plants. This kind of dormancy is caused by a water-impermeable layer that blocks water and oxygen from the surrounding environment and keeps embryos in a viable status for a long time. Most of the work on hardseededness has focused on morphological structure and phenolic content of seed coat. The molecular mechanism underlying physical dormancy remains largely elusive. By screening a large number of Tnt1 retrotransposon-tagged Medicago truncatula lines, we identified nondormant seed mutants from this model legume species. Unlike wild-type hard seeds exhibiting physical dormancy, the mature mutant seeds imbibed water quickly and germinated easily, without the need for scarification. Microscopic observations of cross sections showed that the mutant phenotype was caused by a dysfunctional palisade cuticle layer in the seed coat. Chemical analysis found differences in lipid monomer composition between the wild-type and mutant seed coats. Genetic and molecular analyses revealed that a class II KNOTTED-like homeobox (KNOXII) gene, KNOX4, was responsible for the loss of physical dormancy in the seeds of the mutants. Microarray and chromatin immunoprecipitation analyses identified CYP86A, a gene associated with cutin biosynthesis, as one of the downstream target genes of KNOX4 This study elucidated a novel molecular mechanism of physical dormancy and revealed a new role of class II KNOX genes. Furthermore, KNOX4-like genes exist widely in seed plants but are lacking in nonseed species, indicating that KNOX4 may have diverged from the other KNOXII genes during the evolution of seed plants.

SUBMITTER: Chai M

PROVIDER: S-EPMC4922145 | biostudies-literature | 2016 Jun

REPOSITORIES: biostudies-literature

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A class II KNOX gene, KNOX4, controls seed physical dormancy.

Chai Maofeng M Zhou Chuanen C Molina Isabel I Fu Chunxiang C Nakashima Jin J Li Guifen G Zhang Wenzheng W Park Jongjin J Tang Yuhong Y Jiang Qingzhen Q Wang Zeng-Yu ZY

Proceedings of the National Academy of Sciences of the United States of America 20160606 25

Physical dormancy of seed is an adaptive trait that widely exists in higher plants. This kind of dormancy is caused by a water-impermeable layer that blocks water and oxygen from the surrounding environment and keeps embryos in a viable status for a long time. Most of the work on hardseededness has focused on morphological structure and phenolic content of seed coat. The molecular mechanism underlying physical dormancy remains largely elusive. By screening a large number of Tnt1 retrotransposon- ...[more]

PMID: 27274062

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Project description:BackgroundA water-impermeable testa acts as a barrier to a seed's imbibition, thereby imposing dormancy. The physical and functional properties of the macrosclereids are thought to be critical determinants of dormancy; however, the mechanisms underlying the maintenance of and release from dormancy in pea are not well understood.MethodsSeeds of six pea accessions of contrasting dormancy type were tested for their ability to imbibe and the permeability of their testa was evaluated. Release from dormancy was monitored following temperature oscillation, lipid removal and drying. Histochemical and microscopic approaches were used to characterize the structure of the testa.Key resultsThe strophiole was identified as representing the major site for the entry of water into non-dormant seeds, while water entry into dormant seeds was distributed rather than localized. The major barrier for water uptake in dormant seeds was the upper section of the macrosclereids, referred to as the 'light line'. Dormancy could be released by thermocycling, dehydration or chloroform treatment. Assays based on either periodic acid or ruthenium red were used to visualize penetration through the testa. Lipids were detected within a subcuticular waxy layer in both dormant and non-dormant seeds. The waxy layer and the light line both formed at the same time as the establishment of secondary cell walls at the tip of the macrosclereids.ConclusionsThe light line was identified as the major barrier to water penetration in dormant seeds. Its outer border abuts a waxy subcuticular layer, which is consistent with the suggestion that the light line represents the interface between two distinct environments - the waxy subcuticular layer and the cellulose-rich secondary cell wall. The mechanistic basis of dormancy break includes changes in the testa's lipid layer, along with the mechanical disruption induced by oscillation in temperature and by a decreased moisture content of the embryo.

Dataset Information

A class II KNOX gene, KNOX4, controls seed physical dormancy.

Publications

A class II KNOX gene, KNOX4, controls seed physical dormancy.

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